Field of the Invention
Embodiments of the invention relate to methods and apparatus for recovery of hydrocarbons from geological formations. More particularly, embodiments provided herein relate to recovery of viscous hydrocarbons from geological formations.
Detailed Description
There are extensive hydrocarbon reservoirs throughout the world. Many of these reservoirs contain a hydrocarbon, often called “bitumen,” “tar,” “heavy oil,” or “ultra heavy oil,” (collectively referred to herein as “viscous hydrocarbon”) which typically has viscosities in the range from 100 to over 1,000,000 centipoise. The high viscosity of these hydrocarbons makes it difficult and expensive to produce.
Each viscous hydrocarbon reservoir is unique and responds differently to the variety of methods employed to recover the hydrocarbons therein. Generally, heating the viscous hydrocarbon in-situ, to lower the viscosity thereof, has been employed to enhance recovery of these viscous hydrocarbons. Typically, these viscous hydrocarbon reservoirs would be produced with methods such as cyclic steam stimulation (CSS), steam drive (Drive), and steam assisted gravity drainage (SAGD), where steam is injected from the surface into the reservoir to heat the viscous hydrocarbon and reduce its viscosity enough for production.
However, some of these viscous hydrocarbon reservoirs are located under cold tundra or permafrost layers and may be located as deep as 1800 feet or more below the adjacent land surface. Current methods of production face limitations in extracting hydrocarbons from these reservoirs. For example, it is difficult, and impractical, to inject steam generated on the surface through permafrost layers in order to heat the underlying reservoir of viscous hydrocarbons, as the heat of the injected steam is likely to expand or thaw the permafrost. The expansion of the permafrost may cause wellbore stability issues and significant environmental problems, such as seepage or leakage of the recovered hydrocarbons at or below the wellhead.
Additionally, the current methods of producing viscous hydrocarbon reservoirs face other limitations. One such problem is wellbore heat loss of the steam, as the steam travels from the surface to the reservoir. Wellbore heat loss is also prevalent in offshore wells and this problem is exacerbated as the water depth and/or the well's reservoir depth increases. Where steam is generated and injected at the wellhead, the quality of the steam (i.e., the percentage of the steam which is in vapor phase) injected into the reservoir typically decreases with increasing depth as the steam cools on its journey from the wellhead to the reservoir, and thus the steam quality available downhole at the point of injection is much lower than that generated at the surface. This situation lowers the energy efficiency of the hydrocarbon recovery process and associated hydrocarbon production rates. Further, surface generated steam produces gases and by-products that may be harmful to the environment.
The use of downhole steam generators is known to address the shortcomings of injecting steam from the surface. Downhole steam generators provide the ability to produce steam downhole, prior to injection into the reservoir. Downhole steam generators, however, also present numerous challenges, including high temperatures, corrosion issues, and combustion instabilities. These challenges often result in material failures and thermal instabilities and inefficiencies.
Therefore, there is a continuous need for new and improved apparatus and methods for recovering heavy oil using downhole steam generation with improved thermal efficiency and minimal environmental impact.